1. Field of the Invention
The present invention relates to a solenoid-operated fluid control valve for use in an electronic shift control system of an automotive automatic transmission.
2. Description of the Prior Art
An automatic transmission of automobile includes planetary gear systems disposed between a torque converter and an output shaft. The gears are automatically shifted in response to car speed and engine load by a shift control system having hydraulic actuators adapted to control clutches and bands associated with the planetary gear systems.
Typically, the shift control system is a hydraulic control system designed to control the pressure of an automatic transmission fluid applied to the hydraulic actuators. The traditional hydraulic control system is complicated and includes a 3-way shift valve, a modulator valve responsive to the intake vacuum, a governor responsive to the car speed, and various other hydraulic valves.
More recently, there have been proposed electronic shift control systems having solenoid-operated valves controlled by electronic control units. The advantage of the electronic control systems as compared with the hydraulic systems is that they are less costly and have a wider flexibility in selecting the best gear shift.
In an electronic shift control system, an oil pump in the transmission constantly feeds the pressure line with the automatic transmission fluid under pressure. The solenoid valve signaled by the electronic control unit operates to relieve the line pressure and the modified line pressure is applied to the hydraulic actuators to produce the shift actions. The conventional solenoid valve used in the electronic shift control systems includes a valve member which is movable perpendicular to the plane of the associated valve seat.
The problem encountered in the electronic shift control systems is that, although added with an anti-foam agent, the automatic transmission fluid contains a substantial amount of air bubbles because the fluid is vigorously agitated as it is circulated through the torque converter, gearsets, and multiple-disc clutches of the transmission. It has been found that air bubbles in the automatic transmission fluid give rise to the chattering of the solenoid valve so that the line pressure as controlled by the solenoid valve is considerably deviated from the desired pressure value.
Accordingly, it is an object of the present invention to provide a solenoid valve which is suitable for use in an electronic shift control system of an automotive automatic transmission and which is operable in a stable manner despite the presence of air bubbles in the automatic transmission fluid.
Another problem of the electronic shift control systems is that the automatic transmission fluid is also extremely contaminated by solid foreign materials such as finely divided debris or fragments of metallic materials resulting from wearing of gears and other moving parts as well as finely divided non-metallic debris or particles deriving from the clutch facings and friction linings of the bands.
The ferrous debris or particles born in the fluid are apt to be magnetically attracted to the component parts of the solenoid and would undesirably alter the magnetic path of the solenoid and, hence, the operating property of the solenoid valve. Furthermore, a deposit of sludge consisting of ferrous and non-ferrous materials would alter the travel of the armature and the movable valve member of the solenoid valve.
Therefore, another object of the invention is to provide a solenoid valve for use in an electronic shift control system of an automotive automatic transmission and which is capable of effectively preventing undesirable accumulation of foreign materials.